THE HORSE ANATOMY AND PHYSIOLOGY. 615 



enfeebled, and a fit subject for disease. On the other hand, ex- 

 ercise and taxing the strength of muscle, without regular sup- 

 ply of nutrition to repair waste and wear, impairs and weakens 

 muscular fibre. 



It will be seen that the glorious mean between exertion and 

 rest, moderation and abundance, is to be reached only by an in- 

 telligent understanding of the nature and wants of the animal 

 economy. 



Physiology of Muscle. The movements of the body 

 and limbs are performed mainly by that part known as "flesh" 

 or muscle, which anatomists call "muscular tissue." 



In the half starved "plug," or highly-trained track-horse, in 

 which the fat has almost entirely disappeared, we see nothing 

 but muscles and their tendons, attached to and inclosing the 

 bones beneath the skin-covering of the legs. On the trunk they 

 spread out into layers and folds so as to give symmetry and 

 protect the organs within the trunk, moving freely upon each 

 other, and causing motion of the several parts of the body. 



The tendons are composed of white fibrous tissue, and serve 

 to connect the muscles to the bones. They are stronger than 

 muscle, more compact, and not so easily injured by violence. 

 They center about the joints and occupy less space, than 

 muscle. The fibres are firmly attached to the bones, which at 

 points of union are rough. This union is so strong that it 

 rarely gives way. The bone will break or the tendon snap in 

 the middle, rather than the union of the two loosen. Tendons 

 are non-elastic. 



The muscles are made up of fibres bundled and held in 

 parallel lines by a fine membrane. These fibres under the 

 microscope appear made up of finer fibres, united in linear direc- 

 tion bv a fine filament. Now these finest fibres or fibrilla are 



* 



made up of distinct cells, alternately light and dark. In the 

 action of the muscle, these cells contract, shorten in length in 

 proportion as they increase in width, causing the whole muscle 

 to gain in thickness what it loses in length. It is a most in- 

 teresting fact that this power of contraction and relaxation, 

 called irritability, varies in different individuals according to the 



